Mineral dressing centrifuge

ABSTRACT

A method of mineral dressing and a mineral dressing centrifuge especially suited to the separation and beneficiation of fine metal values such as gold particles from suspensions of mixtures of said metal values and a gangue in a liquid medium. A suspension of the particulate mineral mixture in a suitable liquid medium is introduced generally axially into a rotating centrifuge and separated into its components by the settling rates of the components in the liquid medium under the influence of centrifugal forces. The components are separately discharged in liquid streams issuing from a plurality of discharge outlets and are separately collected. The centrifuge is of general frusto-conical configuration having an axial feed inlet for introducing the suspension into the larger end of the centrifuge and a plurality of axially and circumferentially spaced discharge outlets.

0 lJnlted States Patent 1 1 A Coulson 1 May 1, 11973 [54] MINERAL DRESSING CENTRIFUGE FOREIGN PATENTS OR APPLICATIONS [76] Inventor: Gordon Raymond Coulson, 309-38 1,143,761 2/1963 Germany ..233/27 Avenue S.A., Calgary, Alberta, 241,180 11/1911 Germany ..233/27 Canada i Primary ExaminerGeorge H. Krizmanich [22] Flled: May 1970 Att0rneyArne l. Fors and Frank I. Piper [21] Appl. No.: 39,730

[57] ABSTRACT 52 -U.S. CI ..233/38, 233/44 A meflwd of mineral dressing and a mineral dressing [51]- ..B04b 1/00 g g l i l Suitfd the z f ene iclation 0 me meta va ues suc as go parti- [58] Fleld of cles from suspensions of mixtures of said metal values and a gangue in a liquid medium. A suspension of the 259/96 particulate mineral mixture in a suitable liquid medium is introduced generally axially into a rotating cen- [56] References cued trifuge and separated into its components by the set- UNITED STATES PATENTS tling rates of the components in the liquid medium under the influence of centrifugal forces. The com- 1,014,849 1/1912 Richardson ..233/27 ponents are separately discharged in liquid streams is- ,6 10/1927 na a 59/ suing from a plurality of discharge outlets and are 917,122 1909 -233/22 separately collected. The centrifuge is of general 1,557,672 10/1925 Doemer X frusto-conical configuration having an axial feed inlet 669,732 3/1901 Qhlsson "233/38 for introducing the suspension into the larger end of 585,936 7/1897 Ll the centrifuge and a plurality of axially and circum- 3,407,999 10/1968 Kirkpatrick ..233/l9 R feremiauy Spaced discharge Outlets 9 Claims, 8 Drawing Figures A l i PATENTEIDHAYI 1107s 3, 780.423

SHEET 1 [IF 4 7G 73 ll 37 48 IN VENTOR. GORDON R. COULSON BY amwqms 4 goat mm mm 1 PATENTED SHEET 2 [1F 4 3730 423 VENTOR: R. COULSON GORDON BY I S aw Agent MINERAL DRESSING CENTRIFUGE The present invention relates to mineral dressing centrifuges and to a method of mineral dressing in which such a centrifuge is utilized to beneficiate metal values in a particulate gangue. More particularly, the invention relates to mineral dressing centrifuges which, although useful in a wide range of mineral dressing operations, have specific application in the separation of finely divided gold particles from gold-bearing suspensions. This invention is particularly applicable to the recovery of colloidal and near-colloidal size gold particles from tailings from existing gold separation treatments and from ores, the treatment of which previously was considered not to be economically feasible.

Ores containing finely disseminated metal values often are difficult to process due to slime production which results from fine grinding of the ore necessary to liberate the said values mechanically mixed with gangue minerals. Gold values, for example, are formed in finely disseminated form mechanically mixed with sul-v phides such as arsenopyrite and fine grinding of the ore results in the liberation of colloidal and near-colloidal sized values which are difficult to collect and separate from the fine gangue minerals and which escape from the mill with the tailings. As a result, mill tailings are known to contain considerable gold values which to date have resisted attempts for commercial recovery of the gold.

This invention is based on the finding that effective separation and beneficiation of colloidal and near-colloidal size, e.g., from less than about one to about 500 microns, gold particles from particulate'gangue materials may be effected by centrifuging a slurry in the form of a suspension of such a particulate material mixture in a liquid such as water.

The method of mineral dressing in accordance with the present invention comprises introducing a slurry of a particulate mineral mixture of metal values such as gold values and a gangue in suspension in a liquid medium generally axially into a centrifuge rotating about its axis, said centrifuge having a plurality of circumferentially spaced apart peripheral liquid discharge outlets in a plurality of axially spaced planes, accelerating said slurry to substantially the centrifuge rotational speed whereby said mixture is subjected to centrifugal forces in the centrifuge, concurrently longitudinally moving the slurry through the centrifuge i.e., parallel to the axis of the centrifuge, and separately discharging liquid streams from said peripheral liquid discharge outlets whereby longitudinal travel of the slurry in the centrifuge is effectual for component separation of said mineral mixture between said peripheral liquid discharge outlets.

The present invention also provides a novel mineral dressing centrifuge which comprises a vessel mounted for rotation about its axis and having a plurality of circumferentially spaced apart liquid discharge outlets in a plurality of axially separated planes through its peripheral wall, means for introducing a slurry of a par-' ticulate mineral mixture in a liquidgenerally axially into said centrifuge vessel, means for accelerating said slurry in the centrifuge to substantially the centrifuge rotational sped, means for effecting longitudinal travel of the slurry within the centrifuge, and means for separately collecting liquid streams discharging from each group of axially spaced apart outlets.

It is accordingly a principal object of the present invention to provide a method of mineral dressing and mineral dressing centrifuge by means of which mineral metal values may be effectively beneficiated.

Another object of this invention is the provision of a method of mineral dressing and a mineral dressing centrifuge by means of which near-colloidal size gold particles can be effectively recovered from particulate mineral mixtures containing same in an economic and facile manner.

These and other objects of the invention and the manner in which they can be attained will become apparent from the description of the accompanying drawings, in which:

FIG. 1 is a vertical sectional view through a preferred embodiment of a mineral dressing centrifuge in accordance with the invention;

FIG. 2 is a horizontal plan view of the centrifuge of FIG. 1 partly in section along the line 22 of FIG. 1.

FIG. 3 is an enlarged horizontal sectional view through the centrifuge of FIG. 1 taken along the line 33 of that figure;

FIG. 4 is an enlarged and fragmentary vertical sectional view through the peripheral wall of the centrifuge of FIG. 1 taken along line 44 of FIG. 2 showing an outlet construction;

FIG. 5 is a schematic flow diagram illustrating the use in the method of the invention of the centrifuge shown in FIGS. 1 to 4;

FIG. 6 is a schematic view of another embodiment of the centrifuge in accordance with the invention;

FIG. 7 is a longitudinal section of a further embodiment of the invention; and

FIG. 8 is a transverse section taken along line 8-8 of FIG. 7.

The mineral dressing centrifuge indicated generally at 10 in FIGS. 1-4 of the accompanying drawings comprises a frusto-conical vessel 12 having a peripheral conical wall 14 mounted on base 13 in liquid-tight relation by O-ring 11, and a top member 15 formed integral with wall 14. Nut 9 threaded onto conduit 39 locks top member 15 and wall 14 to base 13. The vessel 12 is mounted for rotation about its longitudinal axis A A by central securement of base 13 to flange 17 of shaft 18 by connecting bolts 21 extending therethrough in a manner to be described. Shaft 18 is journaled for rotation by extension downwardly through upper ball bearing 19 and lower cone bearing 20 supported by housing 22 held on machine frame comprising a top member 24 and upright members 25. The structures of the bearing housing 22 and of the machine frame will not be described herein in greater detail since they comprise merely one technique for rotatably supporting the centrifuge and it will readily be understood by those skilled in the art that numerous types of bearing housings and support frames may be used with the centrifuge of this invention.

Below the bearings 19 and 20, a pulley 28 is keyed to the shaft 18 as at 29 for receiving a driving belt, not shown, which is also trained around a suitable source of motive power. An AC motor operatively connected to a variable drive and a gear reducer has been found to provide a suitable source of motive power with satisfactory speed control.

From FIGS. l and 2, it will be seen that the peripheral wall 14 of the centrifuge vessel 12 has sets of circumferentially spaced apart liquid discharge nozzles 30 extending through wall openings 27 in a plurality of axially spaced apart planes. In particular, it will be seen from FIG. 1 that seven such axial sets of liquid discharge outlets or nozzles 30 are provided through the periphal wall 14 while from FIG. 2 it will be noted that each such set comprises 12 liquid discharge nozzles 30 disposed at angular circumferential separations of 30. For convenience, the seven axial sets of nozzles are identified by the letters B" through I-I" in FIG. 1.

Referring to FIG. 4, it will be seen that each liquid discharge nozzle 30 comprises a tubular portion 32 of reduced cross-section which projects through and extends radially inwardly beyond the inner surface 33 of the peripheral wall 14 and a tubular portion 34 of enlarged cross-section seated within the wall opening 27 such that nozzle shoulder 31 abuts O-ring 35 against wall opening shoulder 36. Nozzle 30 is secured in opening 27 by a lock screw 37 threaded in opening 27. Nozzle 30 and lock screw 37 have aligned axial openings which together define a discharge opening 38. Each discharge opening 38 is of sufficient diameter to ensure passage of solids therethrough without bridging and without an excessive flow of liquid. In the particular embodiment illustrated, the outer surface 26 of the peripheral wall 14 of the centrifuge vesse 12 is shown as having a stepped configuration, but this is not an essential feature of this invention.

The centrifuge also comprises an axially disposed feed conduit 39 which, as shown in FIG. 1, is secured to base 13 by bolts 21 passing through impeller plates 48,50 such that the impeller 49 is disposed at the bottom of the centrifuge, i.e., the enlarged portion of the centrifuge. Conduit 39 is friction-fitted through boss 41 rigidly secured to the top member 15. It will be understood that the feed conduit 39 is secured to the vessel 12 for co-rotation therewith and, to permit control of the rate of introduction ofa slurry into the vessel 12, a non-rotatable conduit portion 44 is freely and telescopically received in the upper end of the conduit 39. O-ring 42 and packing 43 in internally threaded collar 45 provide a water-tight seal between members 39 and 44.

In accordance with a particularly useful feature of this invention, impeller 49 comprising spaced apart circular plates 48,50 is secured to the feed conduit 39 concentric therewith at the open lower end thereof for co-rotation for the purpose of imparting radially outward motion to the particulate suspension introduced into the vesse 12. In the particular embodiment illustrated, plate 50 is welded as at 51 about the lower end of the feed conduit 39.

With reference now to FIG. 3, impeller chamber 54 into which the particulate suspension is discharged from the feed conduit 39 has a plurality of substantially tangential outlets 55 defined between plates 48,50 by wedges 53.

An annular baffle 58 is provided between adjacent sets 8" through H" of discharge nozzles 30. Referring in greater detail to FIG. 4, it will be seen that the provision of such annular baffles 58 leads, on operation of the centrifuge 10, to the buildup ofa bed 59 of particulate mineral material around each of the discharge nozzles 30. This is particularly advantageous in reducing erosion of the inner surface 33 of the peripheral wall 14 ofthe vessel 12.

Axially extending and radially disposed vanes 60, shown most clearly in FIG. 2 and being provided between all adjacent sets of nozzles, extend from abutment with the wall of conduit 39 to the inner surface 33 of wall 14. Each of these radial vanes 60 is retained in position by its engagement in radial slots 62 and 63 formed in the inner surfaces of the base 13 and the top member 15 respectively of the vessel 12. Each of baffles 58 is divided into segments and anchored in slots formed radially in the sides of vanes 60.

The mineral dressing centrifuge 10 also comprises a number of fixed annular collectors or troughs 66B through 661-1 shown somewhat schematically in FIG. 1. Each of these collectors is disposed so as to receive a particulate material suspension stream from a respective one of the sets B" through H of the discharge nozzles 30. Four launder chutes 68 having a wedgeshaped configuration in plan view are provided for draining the particulate material suspension from each of the annular collectors 66 and these chutes discharge at their lower ends into conduits 69 leading to settling troughs 70B through 70H as shown somewhat schematically in FIG. 5 and which overflow into a drainline 71.

The method of the invention and the operation of the mineral dressing centrifuge of FIGS. 1 to 4 will now be described in greater detail with reference to FIG. 5 of the accompanying drawings. From that figure, it will be seen that particulate mineral mixture is fed through a feed line 74 to a grinder mill for reduction therein of the particle size to a desired value to liberate metal values from the gangue. From the mill 76, the resulting ground mixture passes through line 78 to a slurry tank 80 in which it is thoroughly mixed with water fed through a water line 81 to form a slurry of a desired consistency. A perforated deflector plate 82 is provided in this tank 80 to facilitate mixing, and settling in this tank is prevented by a recycle pump 83 and associated lines 84 and 85. I

From the slurry tank 80, the slurry passes through line 86 to a screen classifier 87 for the separation of oversize material which can, if desired, be recycled to the mill 76 through line 88. The material passing through the screen classifier 87 then passes through the feed line 90 to the fixed conduit 44 and into the feed conduit 39 of the centrifuge 10. If desired, additional water may be fed directly to the centrifuge through line 89.

The slurry can be fed to centrifuge 10 under pressure or by a gravity flow utilizing conduit 44 to control the head. Centrifuge 10 can rotate at a speed of from about 200 to about l,000 revolutions per minute (RPM) and the slurry feed pressure relative to the liquid discharge through the nozzles must be adequate to satisfy the discharges from nozzles H-C inclusive and to maintain a liquid discharge from nozzles B. Nozzles B are of sufficient capacity to permit variation of slurry flow.

Slurry discharged into the centrifuge is received in chamber 54 of impeller 49 and impelled tangentially through impeller outlets 55 for acceleration to substantially the centrifuge rotational speed. Vanes 60 then angularly impel the slurry to maintain the slurry rotational speed. Liquid flow is induced upwardly, i.e., longitudinally, by the discharge of liquid from the upper nozzles, i.e., nozzles B. The metal values such as, for example, near-colloidal or colloidal gold, are believed separated from particulate gangue by the method and apparatus of my invention, as will be described in the following examples, due to the influence of centrifugal forces on the solids whereby the coarser gold having a greater specific gravity than the gangue settles first towards nozzles H while the finer gold and gangue is retained in the liquid and is carried upwardly away from nozzles H for discharge from the remaining nozzles according to the solids terminal settling velocities in the liquid.

The following example is described with reference to a centrifuge having an inside bowl diameter of 20 inches and a height of 8% inches with 3/32 inch nozzle openingsin nozzle sets C H inclusive and 5 1 inch nozzle openings in nozzle set B.

EXAMPLE 1 A 10 per cent by weight solids in water slurry was fed to the centrifuge at the rate of 25 GPM for a total of 50.65 pounds of ore solids. The solids from tailings from a gold-arsenopyrite ore, assayed 0.1093 troy ounces gold per ton. The centrifuge was operated at a speed of 300 RPM. Table 1 illustrates the ratio of gold to solids for each nozzle set and the centrifugal forces at the centrifuge periphery.

TABLE I Nozzle %Total %Total Ratio G's Gold Solids Gold2Solids H 32.0 25.15 1:1.28 23.0 G 15.4 14.00 1:1.10 20.4 F 5.3 4.85 121.10 17.9 E 5.4 6.00 120.9 15.3 D 2.9 2.80 121.04 12.8 C 0.8 0.90 120.89 10.2 B 0.7 0.80 120.88 6.6 Overflow 37.5 45.50 120.82

The concentrate from nozzles H was beneficiated 28 per. cent and the products from nozzles F and G were beneficiated 10 per cent.

EXAMPLE II A 10 per cent by weight solids in water slurry was fed to the centrifuge referred to above having inch nozzles in set B. The centrifuge was operated at 200 RPM at a flow rate of 50 GPM. The solids in the slurry assayed 0.12 troy ounces gold per ton. The products from the centrifuge are shown in Table 11.

TABLE II Nozzle %Total %Total Ratio I Gold Solids Gold: Solids H (retained) 10.50 3.57 122.97 G (retained) 1.70 0.87 1:1.95 F-B (retained) 2.16 1.42 121.50 H 4.22 3.98 121.06 0 9.90 7.55 1:1.31 F-C 16.25 1660 1:098 B 8.65 8.80 120.98 Overflow 46.70 57.20 120.81

The concentrate retained in the centrifuge at nozzle H was beneficiated about 200 per cent. The concen trate retained in the centrifuge at nozzle G was beneficiated about 95 per cent and at nozzles F-B about 50 per cent.

EXAMPLE m The liquid overflow from the system in Example 11 containing 0.09 troy ounces per ton gold was returned to the centrifuge under the conditions of Example 11 except that the centrifuge speed was increased to 300 RPM. The products from the centrifuge were as follows in Table III. 1

TABLE 111 Nozzle %Total %Total Ratio Gold Solids Gold2Solids H (retained) 15.08 11.60 121.30 G (retained) 5.78 4.45 121.30 F-B (retained) 6.85 6.85 1:1.00 H-B 27.18 27.46 120.99 Overflow 34.20 38.00 L:0.90

Insufficient solids were present in the centrifuge to indicate beneficiation in the nozzle discharge. However, the centrifuge residues in proximity to nozzles H and G indicated a 30 per cent beneficiation. No beneficiation was evident in the residues at nozzles F-B.

The foregoing Examples indicate beneficiation of the mineral mixture in the products to nozzles H and G, either in the residue or in the nozzle discharge.

Although the description of the Examples has proceeded with reference to a frusto-conical centrifuge having a feed impeller at the base, i.e., wide end, and the truncated portion in an upwards position, it will be understood that the centrifuge of this configuration can be disposed on its side or inverted with the wide end upwards. Also, the feed impeller can be centrally located at the bowl end remote from the wide end. Centrifugal force created by rotation of the centrifuge substantially overcomes the effects of gravity and centrifuge disposition is therefore not critical. The location of the feed impeller at the bowl narrow end remote from the wide end can provide the advantages of increasing centrifugal forces with reduced longitudinal slurry flow velocities as the wide end of the bowl is reached such that the colloidal and near-colloidal solids remaining in the slurry obtain optimum effects from centrifugal forces with maximum retention time.

FIG. 6 illustrates such an embodiment of my invention in which the frusto-conical bowl is disposed with the wide end upwards. The bowl includes a base plate 141 and a cover plate 143 and is journaled for rotation in bearings 142, 144 and rotated by shelf 146 in a manner described with reference to my embodiment described in FlGS. l-4. Impeller 148 receives a slurry from feed conduit 150 and impels the slurry tangentially outwardly towards the periphery of the centrifuge. Nozzles 152, which are disposed circumferentially equispaced about the periphery of the bowl, are arranged in a plurality of axially spaced apart planes and each axial set of nozzles S, T, X, Y is separated from the adjacent set by a baffle 154. Baffle 156 functions as an overflow ring or weir and, in conjunction with the liquid discharge from nozzles 1S2, rate of rotation of the centrifuge and rate of slurry feed, controls the depth of liquid in the centrifuge bowl. The surface of the liquid is designated by numeral 158. Longitudinal vanes 160 angularly impel the liquid at substantially the centrifuge rotational speed.

The operation of this embodiment of my invention is substantially the same as that of the previous embodiment described. Discharge from nozzles 152 is collected in annular collectors 162S, 162T, 162X and 162Y, for discharge through lines 1638, 163T, 163x and 163Y respectively, the discharge from over-flow baffle 156 being discharged from the centrifuge via openings 164 formed circumferentially equispaced about the centrifuge periphery and received in annular collector 166 leading to discharge lines 167.

It will also be understood that the centrifuge configuration is not limited to a frusto-conical shape and can be cylindrical, as shown generally in FIGS. 7 and 8. Cylindrical shell 100 is mounted for rotation at one end 102 by a bearing 104 carrying feed pipe 106 flanged centrally onto end wall plate 108 and at the opposite end 110 by bearing 112 carrying outlet extension 124 flanged onto end wall plate 126. The centrifuge can be rotated by a driving belt, not shown, interconnecting pulley 114 with a source of motive power, also not shown.

An impeller 116 is secured to wall 108 for rotation therewith. In the embodiment shown, a plurality of circumferentially spaced apart peripheral discharge outlet nozzles 118 are arranged in a plurality of axially spaced planes P, Q and R i.e., eight nozzles in each of three sets. Radial vanes 120 are disposed between each pair of adjacent circumferentially spaced nozzles 118 and annular baffles 122 between each adjacent set P, Q and Q,R, as illustrated.

Annular collectors 128, each with an internal radial wall 129 which defines an annular trough 130, are formed about each set P,Q and R of nozzles 118 to receive and remove effluent from the nozzles 118 and to pass said material to discharge lines 132.

In operation the centrifuge is rotated at for example ZOO-1,000 RPM or more. Slurry of the type hereinbefore described is introduced to the centrifuge by feed pipe 106 and tangential impeller 116. The slurry is rapidly accelerated to the centrifuge rotational speed and, in flowing longitudinally through the centrifuge for discharge via overflow ring 124 and via nozzles 118 to be received by collectors 128, is acted upon by the centrifugal forces created. Coarse gold values with high terminal velocities travel to nozzle set P, together with coarse gangue, for a preliminary separation. Finer gold and gangue travel to nozzle sets Q and R and water containing some colloidal material flows over ring 124. The longitudinal flow speed of the slurry can be controlled by the volume ofslurry fed to the centrifuge and by the discharge from the nozzles, the latter being affected by nozzle diameters and Gs of centrifugal force. The paths taken through the centrifuge will vary as to the characteristics of each ore, i.e., size and specific gravity of particulate solids, and centrifuge speed, slurry solids composition, slurry volume, nozzle size, overflow ring size, and depth of annular baffles 122, which can be regulated by the artisan to select a suitable combination of parameters.

Although three axially spaced sets P,Q and R of eight circumferentially spaced nozzles 118 have been shown in FIGS. 7 and 8, and other combinations of, for example, five sets of 12 nozzles each, can be used as is necessary for flow throughput, maximum particle size and nature and degree of separation and beneficiation desired.

Iclaim:

1. A method of mineral dressing which comprises introducing a slurry of a particulate mineral mixture of metal values and a gangue in suspension in a liquid medium, said metal values having a greater specific gravity than the specific gravity of the gangue minerals, generally axially into the large diameter end of a centrifuge having a generally frusto-conical configuration with a large diameter end and a small diameter end rotating about its axis and having a plurality of circumferentially equispaced peripheral liquid discharge outlets in a plurality of axially spaced planes extending from said large diameter end to said small diameter end, accelerating said mixture to substantially the centrifuge rotational speed by impelling the slurry towards the centrifuge periphery whereby said mixture is subject to centrifugal forces, concurrently longitudinally moving the slurry through the centrifuge from the large diameter end towards the small diameter end, and separately discharging liquid streams containing said particulate mineral mixture from said peripheral liquid discharge outlets, whereby longitudinal travel of the slurry in the centrifuge under centrifugal forces is effectual for component separation of said particulate mineral mixture in accordance with terminal velocities of the components of the particulate mineral mixture.

2. A method as claimed in claim 1 in which the slurry is moved longitudinally through the centrifuge by feeding the slurry to the centrifuge at the large diameter end thereof under positive pressure and discharging a substantial portion of the slurry liquid from the centrifuge in proximity to or at the opposite end of the centrifuge.

3. A method as claimed in claim 2 in which the slurry is accelerated to substantially the centrifuge rotational speed by substantially tangentially impelling said slurry towards the centrifuge periphery at one end of the centrifuge and angularly impelling the slurry during its travel through the centrifuge.

4. A method as claimed in'claim 3, capturing solids settling in proximity to nozzles in each axially spaced plane for discharge from said nozzles, and separately collecting the discharge from said nozzles.

S. A novel mineral dressing centrifuge which comprises a vessel having a generally frusto-conical configuration with a large diameter end and a small diameter end mounted for rotation about its axis and having a plurality of circumferentially equispaced liquid discharge outlets through its peripheral wall in a plurality of axially separated planes extending from said large diameter end to said small diameter end, means for introducing a slurry of a particulate mineral mixture in a liquid generally axially into said centrifuge vessel, an impeller mounted concentric with the centrifuge at the large diameter end and adapted to rotate at the centrifuge rotational speed, said impeller having an axial chamber for receiving slurry from the feed pipe and having a plurality of substantially tangential outlets in communication with said chamber for equally distributing slurry towards the centrifuge periphery while accelerating said slurry to substantially the centrifuge rotational speed, means for maintaining the slurry at said centrifugal rotational speed, and means for separately collecting liquid streams discharging from each group of axially spaced apart outlets.

6. A centrifuge as claimed in claim 5 in which said means for introducing the slurry axially into the cenimity to the small diameter end of the centrifuge remote from the feed end adapted to discharge liquid at a rate relative to the slurry feed rate sufficient to maintain a positive longitudinal slurry flow within the centrifuge.

9. A centrifuge as claimed in claim 7 in which an annular baffle is formed within the centrifuge between each adjacent set of axially spaced nozzles. 

2. A method as claimed in claim 1 in which the slurry is moved longitudinally through the centrifuge by feeding the slurry to the centrifuge at the large diameter end thereof under positive pressure and discharging a substantial portion of the slurry liquid from the centrifuge in proximity to or at the opposite end of the centrifuge.
 3. A method as claimed in claim 2 in which the slurry is accelerated to substantially the centrifuge rotational speed by substantially tangentially impelling said slurry towards the centrifuge periphery at one end of the centrifuge and angularly impelling the slurry during its travel through the centrifuge.
 4. A method as claimed in claim 3, capturing solids settling in proximity to nozzles in each axially spaced plane for discharge from said nozzles, and separately collecting the discharge from said nozzles.
 5. A novel mineral dressing centrifuge which comprises a vessel having a generally frusto-conical configuration with a large diameter end and a small diameter end mounted for rotation about its axis and having a plurality of circumferentially equispaced liquid discharge outlets through its peripheral wall in a plurality of axially separated planes extending from said large diameter end to said small diameter end, means for introducing a slurry of a particulate mineral mixture in a liquid generally axially into said centrifuge vessel, an impeller mounted concentric with the centrifuge at the large diameter end and adapted to rotate at the centrifuge rotational speed, said impeller having an axial chamber for receiving slurry from the feed pipe and having a plurality of substantially tangential outlets in communication with said chamber for equally distributing slurry towards the centrifuge periphery while accelerating said slurry to substantially the centrifuge rotational speed, means for maintaining the slurry at said centrifugal rotational speed, and means for separately collecting liquid streams discharging from each group of axially spaced apart outlets.
 6. A centrifuge as claimed in claim 5 in which said means for introducing the slurry axially into the centrifuge comprises a feed pipe disposed substantially concentric with the centrifuge and terminating in proximity to or at one end of the centrifuge interior.
 7. A centrifuge as claimed in claim 5 in which said centrifuge has a plurality of radial longitudinally disposed vanes for angularly impelling the slurry to maintain the slurry of the centrifugal speed.
 8. A centrifuge as claimed in claim 7 in which means are provided for effecting longitudinal travel of the slurry comprising liquid discharge means at or in proximity to the small diameter end of the centrifuge remote from the feed end adapted to discharge liquid at a rate relative to the slurry feed rate sufficient to maintain a positive longitudinal slurry flow within the centrifuge.
 9. A centrifuge as claimed in claim 7 in which an annular baffle is formed within the centrifuge between each adjacent set of axially spaced nozzles. 